Episode 3

A Pakistani agent warns MI5 that an al-Qaeda cell is planning an attack in London, and the team already has an asset inside the cell, which is led by Nadif Abdelrashid. What is supposed to be a dry run turns live, and four bombers are tracked walking through the city towards their final target. The team tries to take them down one by one, and does not entirely succeed.

Lucas North, meanwhile, is having flashbacks to his time being tortured by Russian jailers, and he remembers an interrogator asking about an operation called Sugar Horse. Harry Pearce goes very still. Sugar Horse was one of MI5's most guarded secrets, known only to a very few, and if the Russians know about it, there is a mole somewhere in the senior levels of the intelligence community. He turns to Bernard Qualtrough, a retired spy catcher, to help ferret out the turncoat.

The two halves do not quite fit together. But they both work. The bomber chase is immediate and ugly, all foot traffic and crowded streets, with no clean way to stop a man who is already walking. The Sugar Horse thread is the one that matters, because it points at somebody very senior, and that is a wound the season is going to keep picking at.

Russell Lewis, Ben Richards and David Wolstencroft wrote it, Peter Hoar directed. Peter Firth plays Harry Pearce, Richard Armitage plays Lucas North, and Hermione Norris's Ros Myers, Miranda Raison's Jo Portman, Gemma Jones's Connie James, Hugh Simon's Malcolm Wynn-Jones and Alex Lanipekun's Ben Kaplan are around them. Robert Glenister is the Home Secretary, Ariyon Bakare is Nadif Abdelrashid and Emilio Doorgasingh is Marlin.

The episode works as a reminder of how much of the show runs on trust, and how cheaply it can be spent. Someone at the top has been selling it, and the hour ends with Harry knowing exactly what that means. It is the quietest of the season's questions and the most dangerous one.